The objective of this research is to obtain detailed information about the structure of several membrane transport systems in order to understand the molecular mechanism by which ions are moved across the cell membrane. In the current year we have concentrated on probes which give us information about the transport sites of the human red cell anion exchange system and the (Na,K)-ATPase from dog kidneys. In particular, we have addressed the question of whether sites on different subunits interact during the transport process.